Rock drilling is often carried out by percussion drilling, where a percussion piston, which is often operated hydraulically, is used to create a shock wave with the aid of an impact force that is generated by hydraulic pressure (percussion pressure), the shock wave being transmitted to the drill bit and hence to the rock through the drill steel (drill string). On contact with the rock, pins made of a hard alloy of the drill bit contacting the rock is pushed into the rock, generating a strong enough force to fragment the rock.
In rock drilling of this kind, it is important that the start of the drilling is performed correctly and that drilling is done with care during normal drilling (i.e.
drilling with high impact force) in order to ensure that the drilling takes place in a manner that does not damage the drilling machine/drilling rig.
It applies in general, and especially in the case of drilling under difficult rock conditions and with a strong impact force, that the drill bit should have as good a contact with the rock as possible. A common way of achieving this is to use a piston which works against the drill steel (drill string) and which is usually in the form of a damping piston, which is also used to damp reflexes from the impact of the shock waves against the rock. During drilling, the damping piston is pressed against the drill steel, and the drill steel is thus pressed against the rock, by pressurization of a pressure chamber working against the damping piston. The damping piston is also usually arranged such that, if the damping piston advances too far, i.e. the area in front of the drill steel is soft enough for the impact of the percussion piston to cause the drill steel, and thus the damping piston, to move forwards and past a normal position, an outlet for said pressure chamber is completely or partially opened, resulting in a pressure decrease in the pressure chamber. By detecting this decrease in pressure, the status of the contact with the rock can be determined, and suitable measures can thus be taken.
For example, the percussion pressure can be increased to a normal drilling level when the damping pressure exceeds a defined pressure level, which, for example, can be a pressure level that has been determined as being desirable during normal drilling. Moreover, the percussion pressure can be arranged to be kept at the normal drilling level as long as the damping pressure does not fall below a low-pressure level, which, for example, can be a level that involves lost or poor contact with the rock. If the damping pressure falls below this level, the percussion pressure can be decreased to the start-up drilling level or can be completely shut off. However, this type of control has a number of disadvantages.
For example, there is a considerable risk of idle percussion, i.e. percussion where most of the shock wave is reflected in the drill bit instead of the rock, which leads to a large amount of damaging energy being returned to the drilling machine.
There is therefore a need for an improved method and device for controlling drill parameters, specifically a method and device that at least partially alleviate the problems of the prior art.